1. Field of the Invention
The present invention relates to a wet-type development device and a wet-type image forming apparatus installed in a copier, a printer, a facsimile, or an all-in-one multifunction peripheral for forming a toner image using developer.
2. Description of the Related Art
Japanese Laid-Open Patent Publication No. 2004-008946 (Document 1) discloses a technique in which two rotatable members arranged to face each other are brought into contact with each other to form a thin layer on a surface of one of the rotatable members. The thin layer is formed at a nip section, which is a section where the two rotatable members are in contact with each other.
Referring to FIG. 13, a general wet-type development device will be described. FIG. 13 is a perspective view of a draw-up roller 110 and a conveyance roller 120 included in a general wet-type development device. In the general wet-type development device, a thin layer is formed on conveyance roller 120 using developer W stored in a developer container 148. Developer W includes carrier liquid and toner.
Draw-up roller 110 is provided between conveyance roller 120 and developer container 148 so as to be in contact with conveyance roller 120. Draw-up roller 110 is partially soaked in developer W in developer container 148. Draw-up roller 110 rotates in a direction of an arrow AR110. Conveyance roller 120 rotates in a direction of an arrow AR120 in contact with draw-up roller 110. With the rotation of draw-up roller 110, developer W in developer container 148 is drawn up toward conveyance roller 120.
Draw-up roller 110 is in contact with conveyance roller 120 at a prescribed pressure. Thus, a thin layer of developer is formed at the nip section where draw-up roller 110 and conveyance roller 120 are in contact with each other.
When a thin layer is to be formed on the surface of conveyance roller 120, a streak portion P1 of developer flowing downward in the form of a streak, a raised portion P2 of developer, a lowered portion P3 of developer, and the like may be formed on the surface of draw-up roller 110 as shown in FIG. 13, depending on the shape (diameter or length) of draw-up roller 110, the shape of conveyance roller 120, the kind of carrier liquid included in developer W, the rotational speed of draw-up roller 110, the rotational speed of conveyance roller 120, and the like.
Streak portion P1, raised portion P2, or lowered portion P3 may be produced because developer W accumulates unevenly in the axial direction of draw-up roller 110 in the vicinity of the nip section on the upstream side of the nip section (the front side on the drawing sheet of FIG. 13).
Streak portion P1 is formed such that developer W dripping downward in the form of a streak overlaps developer W drawn upward on the inside thereof whereby developer W is formed like a layer on the surface of draw-up roller 110.
Raised portion P2 is formed such that developer W accumulates in the vicinity of the nip section on the upstream side of the nip section and partially swells. Lowered portion P3 is formed such that developer W is partially lowered in the vicinity of the nip section where developer W is attracted to raised portion P2 and is reduced in amount.
When streak portion P1, raised portion P2, and lowered portion P3 are formed on the surface of draw-up roller 110, thickness variations called streak noise occur in the thin layer formed at the nip section, and the thickness of the thin layer formed on the downstream side from the nip section becomes uneven.